U.S. patent number 5,773,991 [Application Number 08/801,441] was granted by the patent office on 1998-06-30 for motor current sense circuit using h bridge circuits.
This patent grant is currently assigned to Texas Instruments Incorporated. Invention is credited to Ching-Siang Chen.
United States Patent |
5,773,991 |
Chen |
June 30, 1998 |
Motor current sense circuit using H bridge circuits
Abstract
A circuit is used for sensing motor current. This circuit
maximizes the power delivered to the motor whose current is being
sensed. Further, this circuit results in maximum voltage drop
across the motor. This circuit implements a mirrored H-bridge to
mirror a current flowing through a conventional H -bridge having a
sensing resistor in series with a VCM motor. The voltage drop
across the sensing resistor serves as an indication of the current
flowing
Inventors: |
Chen; Ching-Siang (Laguna
Niguel, CA) |
Assignee: |
Texas Instruments Incorporated
(Dallas, TX)
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Family
ID: |
23719367 |
Appl.
No.: |
08/801,441 |
Filed: |
February 18, 1997 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
|
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433236 |
May 2, 1995 |
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Current U.S.
Class: |
324/765.01 |
Current CPC
Class: |
G01R
19/0092 (20130101); H02P 7/04 (20160201) |
Current International
Class: |
G01R
19/00 (20060101); H02P 7/00 (20060101); G01R
023/00 () |
Field of
Search: |
;324/772,713,545,546
;318/490,254,138,439 ;360/73.01 ;361/23,30 ;364/814,550.01
;340/653 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Nguyen; Vinh P.
Attorney, Agent or Firm: Swayze, Jr.; W. Daniel Brady, III;
W. James Donaldson; Richard L.
Parent Case Text
This application is a Continuation, of application Ser. No.
08/433,236 filed on May 2. 1995, now abandoned.
Claims
I claim:
1. A circuit for sensing current in a motor, said circuit
comprising:
a first H bridge circuit having a first arm and a second arm, said
first H bridge, circuit supplying power to said motor, said power
causing a motor current to flow through said motor and said motor
being connected between said first arm and said second arm;
a second H bridge circuit having a third arm and a fourth arm and
supplying power to a sensing resistor, said second H bridge circuit
mirroring a current proportional to said motor current into said
sensing resistor wherein said sensing resistor is connected between
said third arm and said fourth arm; and
a circuit for measuring a voltage across said sensing resistor,
wherein said voltage across said sensing resistor is proportional
to said motor current.
2. The circuit of claim 1 wherein said first H bridge circuit
comprises a first current path including a first transistor coupled
to a power supply in series with said motor and in series with a
second transistor coupled to ground, wherein during a first state
of operation of said circuit current flows from said power supply
to ground through said first current path.
3. The circuit of claim 2 wherein said first H bridge circuit
further comprises a second current path including a third
transistor coupled to said power supply in series with said motor
and in series with a fourth transistor coupled to ground, wherein
during a second state of operation of said circuit current flows
from said power supply to ground through said second current
path.
4. The circuit of claim 1 wherein said second H bridge circuit
comprises a first current path including a first transistor coupled
to a power supply in series with said sensing resistor and in
series with a second transistor coupled to ground, wherein during a
first state of operation of said circuit, a current substantially
equal to said motor current is forced through said sensing
resistor.
5. The circuit of claim 4 wherein said second H bridge circuit
comprises a second current path including a third transistor
coupled to a power supply in series with said sensing resistor and
in series with a fourth transistor coupled to ground, wherein
during a second state of operation of said circuit, a current
substantially equal to said motor current is forced through said
sensing resistor.
6. A circuit for sensing current in a motor, said circuit
comprising:
a first bridge circuit having a first arm and a second arm, said
first bridge circuit supplying power to said motor, said power
causing a motor current to flow through said motor and said motor
being connected between said first arm and said second arm;
a second bridge circuit having a third arm and a fourth arm and
supplying power to a sensing resistor, said second bridge circuit
mirroring a current proportional to said motor current into said
sensing resistor wherein said sensing resistor is connected between
said third arm and said fourth arm; and
a circuit for measuring a voltage across said sensing resistor,
wherein said voltage across said sensing resistor is proportional
to said motor current; and,
wherein said first bridge circuit comprises a first current path
including a first transistor coupled to a power supply in series
with said motor and in series with a second transistor coupled to
ground, wherein during a first state of operation of said circuit
current flows from said power supply to ground through said first
current path; and,
wherein said first bridge circuit further comprises a second
current path including a third transistor coupled to said power
supply in series with said motor and in series with a fourth
transistor coupled to ground, wherein during a second state of
operation of said circuit current flows from said power supply to
ground through said second current path,
wherein said second bridge circuit comprises a first current path
including a fifth transistor coupled to said power supply in series
with said sensing resistor and in series with a sixth transistor
coupled to ground, wherein during said first state of operation of
said circuit, a current substantially equal to said motor current
is forced through said sensing resistor.
7. The circuit of claim 6 wherein said second bridge circuit
comprises a second current path including a seventh transistor
coupled to said power supply in series with said sensing resistor
and in series with an eighth transistor coupled to ground, wherein
during said second state of operation of said circuit, a current
substantially equal to said motor current is forced through said
sensing resistor.
8. The circuit of claim 7 wherein a positive terminal of a
differential amplifier is coupled to a drain of said eighth
transistor, and a negative terminal of said differential amplifier
is coupled to a drain of said fourth transistor, said differential
amplifier causing a voltage of said drain of said eighth transistor
to be substantially equal to a voltage of said drain of said fourth
transistor.
9. The circuit of claim 8 wherein said third, fourth, and eighth
transistors have common gates.
10. The circuit of claim 6 wherein a positive terminal of a
differential amplifier is coupled to a drain of said sixth
transistor, and a negative terminal of said differential amplifier
is coupled to a, drain of said second transistor, said differential
amplifier causing a voltage of said drain of said sixth transistor
to be substantially equal to a voltage of said drain of said second
transistor.
11. The circuit of claim 10 wherein said first, second, and sixth
transistors have common gates.
12. A circuit for sensing current in a motor, said circuit
comprising:
a first stage, said first stage supplying power to said motor, said
power causing a motor current to flow through said motor;
a second stage supplying power to a sensing resistor, said second
stage mirroring a current proportional to said motor current into
said sensing resistor; and
a circuit for measuring a voltage across said sensing resistor,
wherein said voltage across said sensing resistor is proportional
to said motor current;
said first stage comprising a first current path including a first
transistor coupled to a power supply in series with said motor and
in series with a second transistor coupled to ground, wherein
during a first state of operation of said circuit current flows
from said power supply to ground through said first current
path;
said first-stage further comprising a second current path including
a third transistor coupled to said power supply in series with said
motor and in series with a fourth transistor coupled to ground,
wherein during a second state of operation of said circuit current
flows from said power supply to ground through said second current
path;
said second stage comprising a first current path including a fifth
transistor coupled to said power supply in series with said sensing
resistor and in series with a sixth transistor coupled to ground,
wherein during said first state of operation of said circuit, a
current substantially- equal to said motor current is forced
through said sensing resistor;
wherein a positive terminal of a differential amplifier is coupled
to a drain of said sixth transistor, and a negative terminal of
said differential amplifier is coupled to a drain of said second
transistor, said differential amplifier causing a voltage of said
drain of said sixth transistor to be substantially equal to a
voltage of said drain of said second transistor.
13. The circuit of claim 12 wherein said first, second, and sixth
transistors have common gates.
14. A circuit for sensing current in a motor, said circuit
comprising:
a first stage, said first stage supplying power to said motor, said
power causing a motor current to flow through said motor;
a second stage supplying power to a sensing resistor, said second
stage mirroring a current proportional to said motor current into
said sensing resistor; and
a circuit for measuring a voltage across said sensing resistor,
wherein said voltage across said sensing resistor is proportional
to said motor current;
said first stage comprising a first current path including a first
transistor coupled to a power supply in series with said motor and
in series with a second transistor coupled to ground, wherein
during a first state of operation of said circuit current flows
from said power supply to ground through said first current
path;
said first stage further comprising a second current path including
a third transistor coupled to said power supply in series with said
motor and in series with a fourth transistor coupled to ground,
wherein during a second state of operation of said circuit current
flows from said power supply to ground through said second current
path;
said second stage comprising a first current path including a fifth
transistor coupled to said power supply In series with said sensing
resistor and in series with a sixth transistor coupled to ground,
wherein during said first state of operation of said circuit, a
current substantially equal to said motor current is forced through
said sensing resistor;
wherein said second stage comprises a second current path including
a seventh transistor coupled to said power supply in series with
said sensing resistor and in series with an eighth transistor
coupled to ground, wherein during said second state of operation of
said circuit, a current substantially equal to said motor current
is forced through said sensing resistor;
wherein a positive terminal of a differential amplifier is coupled
to a drain of said eighth transistor, and a negative terminal of
said differential amplifier is coupled to a drain of said fourth
transistor, said differential amplifier causing a voltage of said
drain of said eighth transistor to be substantially equal to a
voltage of said drain of said fourth transistor.
15. The circuit of claim 14 wherein said third, fourth, and eighth
transistors have common gates.
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to the field of current sensing in
electrical and electronics circuits. More particularly, the
invention relates to current sensing in the field of electric
motors, such as motors used in computer magnetic recording
apparatus.
2. Background Art
The prior art has attempted to measure the current being passed
through an electric motor. Specifically, the prior art has
attempted to measure the current flowing through Voice Coil Motors
(VCM) and Spindle Motors (SPM). Accurate measurement of the current
passing through these types of electric motors has wide
applicability in computer magnetic recording apparatus. For
example, in disk drive applications, the VCM current must be
controlled so that the VCM can accommodate the positioning of a
servo head on a prescribed track. As another example in disk drive
applications, the SPM current need be well controlled so that the
SPM can rotate platters at a constant speed.
To fulfill the requirement of accurate control of the current
passing through a VCM or a SPM, the current must be accurately
sensed and then fed back to a conventional feed back system. Based
on the magnitude of the feedback current, the speed of the VCM or
SPM is adjusted to meet the requirements of the magnetic recording
apparatus.
FIG. 1 shows a prior art circuit 10 for sensing the current passing
through a VCM (i.e. motor 24 in circuit 10). An NMOS (N-type
Metal-Oxide-Silicon) transistor 14 has its drain connected to Vcc
22 and its source connected to the common node of 29 which is
shared between motor 24, NMOS transistor 14 and NMOS transistor 12.
The drain of NMOS transistor 12 is connected to common node 29, and
the source of transistor 12 is connected to ground 20. NMOS
transistors 18 and 16 form a symmetrical configuration in relation
to NMOS transistors 14 and 12. Transistor 18 has its drain
connected to Vcc 22, while its source is connected to common node
31. Common node 31 is shared between transistors 16, 18, and
sensing resistor 26. The drain of transistor 16 is connected to
common node 31, and the source of transistor 16 is connected to
ground 20.
Motor 24 is a VCM which has one terminal connected to common node
29 and another terminal connected to sensing resistor 26. Sensing
resistor 26 is placed in series with motor 24 so that all of the
current passing through motor 24 passes through sensing resistor
26. Sensing resistor 26 has one terminal connected to motor 24 and
another terminal connected to common node 31. A differential
amplifier 28 is connected to nodes 33 and node 31, i.e. across
sensing resistor 26.
In one state of operation of circuit 10, current flows from Vcc 22
through NMOS transistor 14 and then through motor 24, sensing
resistor 26, and finally through NMOS transistor 16 to ground. This
current path, namely, transistor 14, motor 24, sensing resistor 26,
and transistor 16 forms a first bridge. In another state of
operation of circuit 10, current flows from Vcc 22 through NMOS
transistor 18 and then through sensing resistor 26, motor 24, and
finally through NMOS transistor 12 to ground. A second bridge is
formed by this current path, namely, transistor 18, sensing
resistor 26, motor 24, and transistor 12. In either state of
operation of circuit 10, the current passing through motor 24 flows
through sensing resistor 26, thereby developing a voltage drop
across sensing resistor 26. The voltage dropped across sensing
resistor 26 is amplified by differential amplifier 28. Output 32 of
differential amplifier 28 is used as a feedback signal in a servo
positioning system.
The prior art approach discussed above suffers from several
disadvantages. First, the use of sensing resistor 26 results in
loss of power intended to be delivered to motor 24. This is because
the current passing through motor 24 causes a voltage drop across
sensing resistor 26. The product of the voltage drop across sensing
resistor 26 and the current passing through it represents a power
loss in the system. Second, for low voltage disk drive
applications, where the system operates on a voltage of only 3
volts, it is critical to maximize the voltage provided to motor 24.
Accordingly, any voltage drop across resistor 26 is unacceptable.
Third, when the resistance value of sensing resistor 26 is reduced,
for example to 0.5 ohms, sensing resistor 26 becomes expensive to
manufacture. Moreover, as the resistance of sensing resistor 26 is
lowered, the size of resistor 26 increases, thus taking up a large
area on a printed circuit board.
Therefore, there is need in the art to overcome the shortcomings of
the prior art motor current sensing circuits, such as circuit 10 in
FIG. 1.
SUMMARY OF THE INVENTION
The present invention discloses a circuit that overcomes the
disadvantages of prior art motor current sense circuits. The
invention maximizes the power delivered to the motor whose current
is being sensed. Further, the invention results in maximum voltage
drop across the motor. The invention overcomes the prior art need
to manufacture sensing resistors with ultra-small values. The
invention also overcomes the prior art need to use external sensing
resistors. This results in savings of area on printed circuit
boards and also a reduction in the cost of manufacturing resistors
with ultra-low resistivity.
The invention achieves the above results by mirroring the motor
current onto a mirrored bridge. The current that flows through the
mirrored bridge is a small fraction (for example, 0.1%) of the
motor current. This reduction in current allows the value of the
sensing resistor across the mirrored bridge to be reasonable. The
value. of the sensing resistor is such that a) it is easily
implemented in an integrated circuit, and b) its absolute tolerance
with process variations is insignificant since it (the sensing
resistor) is made to match a feedback resistor in order to cancel
the effects of process variations. Accordingly, the prior art
requirement that the sensing resistor be in series with the motor
is overcome.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 shows a prior art motor current sense circuit.
FIG. 2 shows an embodiment of a current sense circuit according to
the present invention.
FIGS. 3A through 3D show computer simulation results illustrating
the operation of the embodiment of the present invention shown in
FIG. 2.
FIG. 4 shows computer simulation results further illustrating the
operation of the embodiment of the present invention shown in FIG.
2.
FIG. 5 shows another embodiment of a current sense circuit
according to the present invention.
DETAILED DESCRIPTION OF THE PRESENT INVENTION
A precision motor current sense circuit is described. In the
following description, numerous specific details are set forth in
order to provide a more thorough understanding of the present
invention. It will be apparent, however, to one skilled in the art,
that the present invention may be practiced without these specific
details. In other instances, well known features have not been
described in detail so as not to obscure the present invention.
FIG. 2 shows an embodiment of the precision motor current sense
circuit 40 of the present invention. In this embodiment, all
transistors are Ntype MOS transistors, except for PMOS transistors
54 and 56. Transistors 42, 44, 46, and 48 form an "H-bridge" (also
referred to as a "first stage" in the present application) to
provide a current through a VCM 24 of circuit 40, while transistors
50, 52, 54, 56, sensing resistor 58, and amplifiers 60, 62, and 64
form a "mirrored H-bridge" (also referred to as a "second stage" in
the present application) of circuit 40.
Turning attention to the H-bridge of circuit 40, the drain of NMOS
transistor 42 is connected to Vcc 22, and the source is connected
to node VM2. Vcc 22 is connected to external power supply through
appropriate pins of the IC package that houses the invention's
current sense circuit 40. Node VM2 is a common node shared between
transistor 42, a first terminal of motor 24, and transistor 44.
NMOS transistor 44 has its drain connected to node VM2 and its
source connected to ground 20. Ground 20 is connected to external
ground (not shown) through appropriate pins of the IC package. The
drain of NMOS transistor 46 is connected to Vcc 22, and its source
is connected to common node VM1. Node VM1 is shared between the
source of transistor 46, a second terminal of motor 24, and the
drain of NMOS transistor 48. The source of transistor 48 is
connected to ground 20. Motor 24 has one terminal connected to VM2
and another terminal connected to VM1.
Turning attention to the mirrored H-bridge of circuit 40, P-type
MOS transistor 54 has its source connected to Vcc 22, and its drain
connected to node VS2. Node VS2 is a common node shared between the
drain of transistor 54, the positive terminal of differential
amplifier 60, the negative terminal of differential amplifier 64, a
first terminal of sensing resistor 58, and the drain of NMOS
transistor 50. Transistor 50 has its drain connected to common node
VS2, and its source connected to ground 20. The source of PMOS
transistor 56 is connected to Vcc 22, and the drain of transistor
56 is connected to node VS1. Node VS1 is a common node shared
between the drain of transistor 56, the positive terminal of
differential amplifier 62, the positive terminal of differential
amplifier 64, a second terminal of sensing resistor 58, and the
drain of NMOS transistor 52. Transistor 52 has its drain connected
to common node VS1, and its source connected to ground 20.
Sensing resistor 58 has one terminal connected to common node VS1,
and another terminal connected to common node VS2. Differential
amplifier 60 has its positive terminal connected to common node
VS2, while its negative terminal is connected to common node VM1 of
the H-bridge of circuit 40. Differential amplifier 62 has its
positive terminal connected to common node VS1, while its negative
terminal is connected to common node VM2 of the H-bridge. The
output of differential amplifier 60 is connected to gate 61 of PMOS
transistor 56, while the output of differential amplifier 62 is
connected to gate 63 of PMOS transistor 54. Differential amplifier
64 has its positive terminal connected to common node VS1, and its
negative terminal connected to common node VS2. NMOS transistors
52, 46, and 44 share a common gate 65, while NMOS transistors 50,
48, and 42 share a common gate 67. Output 66 of differential
amplifier 64 is connected to feedback resistor 68. Feedback
resistor 68 is in turn coupled to an error amplifier (not shown)
which is coupled to motor 24.
In a first state of the operation of current sense circuit 40, a
current path is formed by the current flowing from Vcc 22 to
transistor 42, through motor 24 and transistor 48 to ground. This
current path forms a first bridge in the H-bridge of circuit 40. In
a second state of the operation of circuit 40, a second current
path is formed by the current flowing from Vcc 22 to transistor 46,
through motor 24, and finally through transistor 44 to ground. This
current path forms a second bridge in the H-bridge of circuit
40.
In the first state of the operation of current sense circuit 40,
the current passing through the first bridge of the H-bridge,
namely, the current passing through transistors 42 and 48 is
"mirrored" by transistor 50 of the mirrored H-bridge of circuit 40.
In the second state of the operation of current sense circuit 40,
the current passing through the second bridge of the H-bridge,
namely, the current flowing through transistors 46 and 44 is
"mirrored" by transistor 52 of the second stage of circuit 40.
In order for transistor 50 to accurately mirror the current in the
first bridge of the H-bridge, two conditions must be met. First,
the gate voltage of transistor 50 must be equal to the gate voltage
of transistors 42 and 48. In circuit 40 this is accomplished
because transistors 50, 42, and 48 share a common gate. The second
condition is that the drain voltage of transistor 50 (i.e. the
voltage at node VS2) must be equal to the drain voltage of
transistor 48 (i.e. the voltage at node VM1). If this second
condition is met, and assuming that transistor 50 has the same size
as transistor 48, the current flowing through transistors 50 and 48
will be equal. Accordingly, the current flowing through transistor
50 will be equal to the current flowing through the first bridge of
the H-bridge of circuit 40.
To meet the second condition, a high gain differential amplifier 60
is used with voltages VS2 and VM1 as inputs to its positive and
negative terminals, respectively. High gain differential amplifier
60 forces the voltages at nodes VM1 and VS2 to be equal. When, the
voltage at node VS2 is higher than the voltage at node VM1, the.
output of differential amplifier 60 transitions high. This causes
an increase in the voltage of gate 61 of PMOS transistor 56. Since
the gate to source voltage of transistor 56 is reduced, the current
flowing through transistor 56 is proportionately decreased.
Accordingly, the current flowing from Vcc 22 through the current
path formed by transistor 56, sensing resistor 58, and transistor
50 decreases. In other words, the path from node VS2 to Vcc 22
becomes more resistive. It follows that the voltage at node VS2 is
forced closer to ground voltage. This reduction of the voltage at
node VS2 is continued until the voltages at nodes VM1 and VS2
become equal. In a similar manner, when the voltage at node VS2 is
less than the voltage at node VM1, the voltage at node VS2 is
forced higher until the voltages at nodes VM1 and VS2 become equal.
Thus, transistor 50 forces the current in the current path formed
by transistor 56, sensing resistor 58, and transistor 50 to be
equal to the current in the first bridge of the H-bridge. The path
formed by transistor 56, sensing resistor 58, and transistor 50
comprises a first bridge of the mirrored H-bridge of circuit
40.
With respect to the second state of the operation of circuit 40,
transistor 52 must mirror the current passed through transistors 46
and 44 which comprise the second bridge of the H-bridge of circuit
40. For transistor 52 to accurately mirror this current, two
conditions must be met First, the gate voltage of transistor 52
must be equal to the gate voltage of transistors 46 and 44. In
circuit 40 this is accomplished because transistors 52, 46, and 44
share a common gate.
The second condition is that the drain voltage of transistor 52
(i.e. the voltage at node VS1) must be equal to the drain voltage
of transistor 44 (i.e. the voltage at node VM2). If this conditions
is met, and assuming that transistor 52 has the same size as
transistor 44, the current flowing through transistors 52 and 44
will be equal. Accordingly, the current flowing through transistor
52 will be equal to the current flowing through the second bridge
of the H-bridge of circuit 40. To meet the second condition, a high
gain differential amplifier 62 is used with voltages VS1 and VM2 as
inputs to its positive and negative terminals, respectively. High
gain differential amplifier 62 forces the voltages at nodes VM2 and
VS1 to be equal. When, the voltage at node VS1 is higher than the
voltage at node VM2, the output of differential amplifier 62
transitions high. This causes an increase in the voltage of gate 63
of PMOS transistor 54.1 Since the gate to source voltage of
transistor 54 is reduced, the current flowing through transistor 54
is proportionately decreased. Accordingly, the current flowing from
Vcc 22 through the current path formed by transistor 54, sensing
resistor 58, and transistor 52 decreases. In other words, the path
from node VS1 to Vcc 22 becomes more resistive. It follows that the
voltage at node VS1 is forced closer to ground voltage. This
reduction of the voltage at node VS1 is continued until the
voltages at nodes VM2 and VS1 become equal. In a similar manner,
when the voltage at node VS1 is less than the voltage at node VM2,
the voltage at node VS1 is forced higher until the voltages at
nodes VM2 and VS1 become equal. Thus, transistor 52 forces the
current in the current path formed by transistor 54, sensing
resistor 58, and transistor 52 to be equal to the current in the
second bridge of the H-bridge. The path formed by transistor 54,
sensing resistor 58, and transistor 52 comprises a second bridge of
the mirrored H-bridge of circuit 40.
According to an embodiment of the invention, transistors 50 and 52
are scaled down versions of transistors 48 and 44, respectively.
For example, transistors 50 and 52 can be few hundred times smaller
than transistors 48 and 44, respectively. Thus, the current
mirrored into the first bridge of the mirrored H-bridge is a few
hundred times smaller than the current flowing in the first bridge
of the H-bridge. Likewise, the current mirrored into the second
bridge of the mirrored H-bridge is a few hundred times smaller than
the current flowing in the second bridge of the H-bridge.
Accordingly, sensing resistor 58 can also be a few hundred times
larger than the prior art sensing resistors. Differential amplifier
64 is a means for measuring the voltage across sensing resistor 58.
Output 66 of differential amplifier 64 is an indication of the
voltage difference between nodes VS1 and VS2, and hence an
indication of the current flowing through sensing resistor 58.
Output 66 is connected to feedback resistor 68, thus the voltage at
output 66 causes a current to flow through the feedback resistor
68. Accordingly, the current through feedback resistor 68 tracks
the current flowing through sensing resistor 58. Feedback resistor
68 and sensing resistor 58 are fabricated by the same material and
process steps so that process variations affect both resistors in
the same manner. Thus, the current flow through feedback resistor
68 tracks the current flow through sensing resistor 58 regardless
of process variations. The current through feedback resistor 68 is
eventually fed to an error amplifier (not shown in the Figures).
The error amplifier compares the current through feedback resistor
68 to a VCM command current (i.e. the desired operating current of
motor 24). Thus, knowing the current flowing through motor 24 (by
knowing the current through feedback resistor 68), the current
through motor 24 is adjusted in order to match the desired
operating current.
FIGS. 3A through 3D show results of computer simulation for the
embodiment of the invention discussed above. In these computer
simulations, motor 24 is modeled by a constant resistor. Although,
the simulation model used for a motor, for example a VCM, is
typically an inductor in series with a resistor, in steady state
conditions the inductor acts substantially as a short circuit.
Accordingly, a single invariable resistor was used to accurately
model motor 24 in the computer simulations that resulted in the
waveforms shown in FIGS. 3A through 3D. FIG. 3A shows the waveform
representing the voltage difference between VS1 and VS2 (i.e. the
voltage across sensing resistor 58). FIG. 3B shows the waveform
representing the voltage difference between VM1 and VM2 (i.e. the
voltage across motor 24). As shown in FIGS. 3A and 3B, the voltage
across sensing resistor 58 closely and accurately tracks the
voltage across motor 24.
FIG. 3C shows the waveform representing the current passing through
the first bridge of stage 2, namely i(50), and the second bridge of
the second stage, namely i(52). As shown, these currents alternate
during the first and second states of the operation of circuit 40
of the invention. FIG. 3D shows the waveform representing the
current passing through the first bridge of the first stage,
namely. i(48), and the current passing through the second bridge of
the first stage, namely i(44). As shown in FIGS. 3C and 3D, the
currents in the first bridges of the first stage and the second
stage closely track each other, except that the current mirrored
into the second stage is one hundred times smaller than the current
in the first stage. Likewise, the currents in the second bridges of
the first stage and the second stage closely track each other,
except that the current mirrored into the second stage is one
hundred times smaller than the current in the first stage.
FIG. 4 shows the voltage difference between VM1 and VM2 (i.e. the
voltage across motor 24) on the x-axis, and the voltage difference
between VS1 and VS2 (i.e. the voltage across sensing resistor 58)
on the y-axis. As shown in FIG. 4, with appropriate values for
sensing resistor 58, the voltage across sensing resistor 58
accurately and closely tracks the voltage across motor 24. It
follows that the current flowing through sensing resistor 58 also
accurately tracks the current flowing through motor 24.
FIG. 5 shows another embodiment of the present invention. In this
embodiment, sensing resistor 58 of FIG. 2 is removed from across
nodes VS1 and VS2. Instead, sensing resistors 58A and 58B, each of
which have the same resistance as sensing resistor 58 of FIG. 2,
are placed in series with transistors 54 and 56, respectively. This
embodiment of the invention works similarly to the embodiment of
FIG. 2. In this embodiment, the voltage difference between nodes
VS1 and VS2 is an indication of the current flowing through sensing
resistors 58A and 58B. Output 66 of differential amplifier 64
amplifies the difference between voltages VS1 and VS2. Output 66 is
an indication of the current flowing through sensing resistors 58A,
during the first state of the operation of circuit 70 of FIG. 5.
Likewise, output 66 is an indication of the current flowing through
sensing resistor 58B, during the second state of the operation of
circuit 70 of FIG. 5. One advantage of this embodiment is that the
fluctuations of voltage at the inputs of differential amplifier 60
are referenced with respect to VCC 22 which is fixed voltage.
Accordingly, the design of differential amplifiers 60 and 62 for
small signal operation is relatively easy because of the
improvement in the common mode range of the amplifiers.
The present invention overcomes the prior art problems that
resulted from placing a sensing resistor in series with the motor.
As stated above, sensing resistor 58 of the embodiment of FIG. 2
(or sensing resistors 58A and 58B of the embodiment of FIG. 5) can
be a few hundred times larger than the prior art sensing resistor.
Thus, the sensing resistor of the invention has a much more
reasonable value than the ultra-small sensing resistor of the prior
art. Accordingly, the sensing resistor of the invention presents a
significant reduction in cost over the sensing resistor of the
prior art. Further, the sensing resistor of the invention consumes
a smaller silicon area, and can in fact be implemented in an IC.
Moreover, the various embodiments of the invention do not take
power away from the motor whose current is being measured. Also,
the invention does not result in a loss of voltage available to the
motor. This advantage of the invention is particularly valuable in
low voltage disk drive applications.
Although the invention has been described with reference to
specific embodiments, it is appreciated by those skilled in the art
that changes in various details may be made without departing from
the invention defined in the appended claims. For example, the MOS
transistors can be replaced by bipolar or other types of
transistors. Further, N-type MOS transistors may be replaced by
P-type MOS transistors with appropriate changes in voltage
polarities. As another example, the present invention can be
applied to sense the current in any kind of an electric motor,
including VCM's and SPM's.
Thus, a precision motor current sense circuit has been
described.
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